The present invention relates to a multilayer film comprising a layer (a) and a layer (c) based in each case on an amount of at least 30% by weight of at least one propylene copolymer and on an amount of at least 20% by weight of at least one propylene homopolymer, and optionally on a further polyolefin, preferably polyethylene, and optionally up to 30% by weight of additives, based in each case on the total weight of the layer (a) respectively (c), where said components always give a total of 100% by weight, at least one internal layer (b) based on at least one homo- and/or polyamide having isophoronediamine units as polyamide component, and, in each case, arranged between the layers (a) and (b), respectively (b) and (c) an adhesion-promoter layer (d) respectively (e) in each case based on at least one modified thermoplastic olefin homo- or copolymer, to a sealing sheet comprising a multilayer film of this type, and also to the use thereof for covering roofs.
The prior art discloses a wide variety of multilayer films which are suitable for a wide variety of technical applications. However, a requirement for some of these technical applications is that the multilayer films used for these purposes feature a combination of quite particular mechanical properties, and also specific barrier properties. This particularly applies to multilayer films which are used for producing sealing sheets, in particular bitumen-based sealing sheets. Bitumen-based sealing sheets of this type are used by way of example for covering roofs.
The prior art has already disclosed sealing sheets of this type comprising multilayer films. However, a disadvantage of said sealing sheets produced of multilayer films is that these often lack adequate barrier properties with respect to oils, in particular mineral oils. Oils of this type are in particular components of bitumen-based compositions. Migration of said oils from the bitumen-based composition into the multilayer film causes not only drying-out of the bituminous composition but also a reduction in the adhesion of the multilayer film on the bitumen-based sheets resulting in a separation thereof from the bitumen-based composition over the course of time.
The multilayer films disclosed in the prior art and used for producing sealing sheets moreover often have unsuitable mechanical properties, for example inadequate tensile strength, poor tear-propagation properties, and insufficient puncture resistance. Specifically, multilayer films for sealing sheets, e.g. for covering roofs, have to comply with stringent mechanical requirements because they have exposure to high mechanical loads resulting from personnel walking thereon, and from storage and movement of construction material, and also from effects of weathering.
There is therefore a need for multilayer films which exhibits not only very good barrier properties with respect to oils but also very good mechanical stability.